I tested the timing by timing light and it is perfect. I use idle advance so it is around 18 deg. Injector deadtime should be OK, I have bosch Greentops, attached the datasheet. I set it to 0.765ms.

Yes, the misfire happen between 2000 and 3000 rpm and only when engine is hot (may the warmup fuel or cold engine head has some positive effect on it.

It is also strange that sometimes at some rpm my engine is happy for couple of seconds by 14.4 AFR sometimes not. Today I also noticed that I cruised around 3000rpm, engine was hot, I ran 13.6 AFR but when misfire occured I could see on the LC1 display as well that it jumps to like 14.4 AFR. I have AFR correction OFF now.

It is LC1 and calibrated. I will do a free air calibration today to see if something change. What do you think do I need to remove the sensor from the pipe or just clean with some high presure air the exhaust from the intake side? (blow tru the system).

its worth blowing the sensor out with an airline too befor you cal it,

as your CR is low i presume this is a blown engine? what are you doing for intake? single throttle and plenum? how big? wats the runner diameter, engine cc, cam profile?

the reason I ask is I'm getting somthing similar, a stumble at cruise, think mine is due to an aggressive cam and large plenum, in my case 274 degrees seat timing, but 240 degrees @50 thou, which is more like the figure ecpected on a 290 degree cam on my engine... due to this there is a lot of lift on overlap, and i supect the 50kpa in the plenum is pulling the exhaust back into the chamber....

My plan is to swap to ITB's which should damp this effect, failing that I'll change the cam back to what was in there before.

I attached a pics about my intake including blower. The main pipe after the blower is 70mm, the throttle is 55mm, after the intercooler I have 4x 38mm ID until the 45mm intake valves. The exhaust valves are 40mm.

Before I changed from the factory Motronic to Megasquirt I never had problems during cruising.

The Megasquirt engine setup is identical from mechanical point of view to the Motronic one.

The only missing thing is the AFM before the blower and I changed from 265cc injectors to Bosch 440cc ones and running sequential.

I had no chance to make free air cal today but will do and let you know if something changed.

Joe Clark wrote:its worth blowing the sensor out with an airline too befor you cal it,

as your CR is low i presume this is a blown engine? what are you doing for intake? single throttle and plenum? how big? wats the runner diameter, engine cc, cam profile?

the reason I ask is I'm getting somthing similar, a stumble at cruise, think mine is due to an aggressive cam and large plenum, in my case 274 degrees seat timing, but 240 degrees @50 thou, which is more like the figure ecpected on a 290 degree cam on my engine... due to this there is a lot of lift on overlap, and i supect the 50kpa in the plenum is pulling the exhaust back into the chamber....

My plan is to swap to ITB's which should damp this effect, failing that I'll change the cam back to what was in there before.

You need to find out what is the intake overlap of your setup, you can greatly improve on the exhaust gas reversing issue by getting a header with stepping size and you will gain a lot power. Taper on the intake runner also can helps a lot to hold the intake charge not reversing flow back to plenum

Today I had the chance to recalibrate the sensor. (Sensor was removed from the pipe and free air calibration was done.)

After calibration I started VEAL to have the VE table corrected based on the new O2 calibration. There was only some slight change in the new VE table was generated by autotune.

So, the Wideband calibration did not solve the problem. Ve table remained aroud the same.

After this I started to play with my injector deadtime. My deadtime based on datasheet is 0.76ms but the min. pulsewidth is 1.13ms. I changed to deadtime to 1.1ms and the misfire issue became better. (I had 4 acceleration from 2000 to 3000rpm from 10 trials without misfire). (Also, engine was running better, like at 14.5 AFR I had no misfire while cruising, engine ran smooter, only some hesitation at 14.5AFR like somebody is pushing back the car for a while. But still had some misfire during acceleration.)

I know the 1.13ms is not the deadtime of me injectors what I verified also by using the following : with my old bosch 265cc injectors I had values around 38 in my idel VE table, also around 35 with the new Bosch greentop when I used 0.765ms deadtime. As soon as I changed the deadtime to 1.1ms I had values around 28 in the idle VE table which showed the deadtime is incorrectly set since VE table shows huge differences compared to the 265cc setup.

However, in case I changed the deadtime to 1.1ms the engine ran better (still not smooth.)

After I changed back the deadtime to 0.765ms I started to play with the ignition table. I noticed when I increase advance I have again some better running engine.

Does any of you can get any conclusion from the above as far as possible root cause of my misfire?

I was playing more than a week with my misfire issue again. With the current tune (I attached) after I was playing with the following parameters:

- Injection timing from 250deg to 500 deg in 10 deg steps- AFR table changes: tried leaner and richer as well in the full region- running in sequential, semi seq and seq. off modes- VVT activation modes from 1000RPM to 4000rpm, VVT OFF (it is a 2 phase VVT)

The issue is still there.

The misfire (a spot where engine running not smooth but lumpy for one or two spark, as Kennyx said maybe some charge back into the intake, a stumble) occurs in 10% when cruising (do not care that it is 12.5AFR 9or 14.8 AFR), but definitely happen when we are on medium load (75 - 80 kPa MAP and like going uphill at moderate throttle). If I use my 1st gear until 4000rpm and upshift to 2nd and apply 50% throttle, that there is no stumble, but when I use 1st until 2000rpm and upshift to 2nd and accelerate by some 25% throttle till 3500 than for sure I will have the symptom around 2500 - 3000 rpm.)